Conventionally, a structure such as that disclosed, for example, in Patent document 1 (see below) is known for a seat pad that is mounted in a vehicle and is used as a seating portion.
In this type of seat pad, when the vehicle is travelling around a gentle curve, or when the vehicle changes lanes while traveling along a road, vibration in a low frequency band of around approximately 1 Hz is input into the mounted seat pad. At such times, as a result of the shape of the seat pad becoming deformed, there may be cases in which the vehicle occupants feel a sensation of instability such as a sideways slipping or a sideways roll around the roll axis of the vehicle. In particular, in a thinly formed seat pad, taking into account flexure, springiness, and bottoming out, generally, a material having high cushioning properties (i.e., springback) is employed. Because of this, it is easy for the stability of the seat surface to be lost, and for the seat occupant to experience a sensation that the seat is unstable.